A. Field of the Invention
This invention relates generally to the field of focal plane shutters found in cameras used for aerial reconnaissance, aerial surveying, mapping and other similar applications.
B. Description of Related Art
Focal plane shutters are known in the art. They are commonly found on aerial reconnaissance cameras, including cameras based on a film image-recording medium and cameras based on an electro-optical image recording medium, such as a charge-coupled device. The shutters typically use an opaque material known as a xe2x80x9ccurtainxe2x80x9d, or a pair of curtains between which a gap or space is formed. The curtain or curtains are selectively and controllably moved relative to a photosensitive image recording medium to thereby control the exposure of the medium to radiation from a scene of interest.
Prior art reconnaissance camera shutters developed in the early 1960""s were mostly mechanical by design. The shutter was comprised of two adjacent curtains placed in the optical light path of the camera. Driven by a series of gears, cams, and cam followers, the curtains performed the shutter function by moving simultaneously, but slightly separated from each other, across the focal plane. The scene presented to the camera was exposed on film by rapidly moving the open xe2x80x98slitxe2x80x99 formed, by the separated curtains, across the film. The width of the slit was adjusted to determine the desired exposure.
One such shutter, manufactured by Recon/Optical, Inc., was known as the KS-87 shutter and found in many reconnaissance cameras of that era. It used what is called the xe2x80x98window shadexe2x80x99 principle. The shutter curtains were xe2x80x98cockedxe2x80x99 in the closed position, and held by an electromagnetic brake. When the exposure command was given, the brake was released and the shutter curtains traversed the optical opening of the focal plane exposing the film to the scene radiation similar to a window shade being unlocked and rolling upwards to the roller. The shutter was then re-cocked and the curtains drawn back to the start or closed position. An obvious disadvantage was that the shutter curtains had to be cocked and held by clutches before each shutter operation. The use of springs, gears, cams, and other mechanical complexities resulted in a low shutter cycle life expectancy, or MTBF (mean time between failure).
In the mid-1980""s an improved shutter, referred to as the KS-147 shutter, was developed by Recon/Optical, Inc. The shutter is described in the patent to Hughes et al., U.S. Pat. No. 4,664,494, assigned to the Assignee of the present invention. The entire content of the Hughes et al. patent is incorporated by reference herein. Although the KS-147 shutter still used two curtains and an associated moving slit as the exposure method, the shutter operated on an entirely different principle from the older KS-87 shutter design. In particular, the curtain assembly was now moved across the focal plane at a controlled rate using a pair of motors, one for each curtain. The curtains were started and then stopped by the motors after the exposure was completed. No springs were necessary to xe2x80x9ccockxe2x80x9d and move the curtains. When the subsequent exposure command was given, the curtains were moved by the motors in the opposite direction across the optical opening and again brought to a stop. This technique is referred to as bi-directional shutter operation.
The KS-147 shutter was developed for five (5) inch film format reconnaissance cameras. The shutter curtains are driven at a speed of 100 inches per second by a set of timing belts using pulleys and D.C. brush type motors. The relative position of the two shutter curtains, with respect to each other, is maintained by using a curtain position sensing potentiometer coupled to the drive motor via a gearbox. To minimize variation during an exposure, the curtain drive belts are mechanically locked by an electromechanical clutch. This clutch is activated at the beginning of the exposure cycle and de-activated at the end of the exposure cycle to permit changes in exposure time if required due to changing light conditions. The relative curtain positioning, which determines the slit width and exposure, utilizes single turn potentiometers to sense curtain position. This positioning requires mechanical adjustment during the initial manufacture and calibration of the shutter.
The KS-147 shutter uses both analog and digital electronics technology. The shutter operating parameters such as curtain acceleration and deceleration are factory pre-programmed into an electronically programmable read only memory chip or EPROM. The EPROM signal is converted to an analog signal, which represents the shutter parameters, by a digital to analog converter (DAC). The output of the DAC is then summed with the output of the curtain position sensing potentiometers in the shutter. The combined signal is used to develop the D.C. voltage required to drive the D.C. torque motors and in turn the shutter curtains.
Although it was acceptable at the time of development, this type of design has several drawbacks. First, in order to change the system parameters, the EPROM must be removed and reprogrammed externally, or the EPROM must be replaced with one containing new parameters. Second, the brushes in the D.C. torque motors generate noise and are subject to wear. Third, the wiper used in the potentiometer is also subject to wear resulting in noise and subsequent poor shutter performance. The maximum cycle rate of the KS-147 shutter using this design is only 4 cycles per second with an average life cycle expectancy of only 200,000 cycles.
Both the KS-87 and KS-147 shutters were designed for five (5) inch format film reconnaissance cameras and, at the time, filled that requirement well. The number of shutter cycles required by the camera was modest and film was the preferred medium. However, with the advent of electro-optical (EO) cameras and the use of solid-state electronic imaging arrays in place of film, the need for a smaller EO format shutter with increased reliability over an increased number of cycles became necessary. As the demand for electro-optical cameras has increased, so has the need for a high cycle life, higher frame rate shutter.
The prior art shutters described above do not meet the described needs, thereby providing the impetus for the present invention. In particular, the art has needed a shutter which will operate at faster rates, for a longer life cycle and greater MTBF, and with a longer period of time between maintenance, exceeding the capability of the KS-147 shutter. In particular there is a demand for a compact, robust, long-life shutter capable of operating at 7xc2xd frames per second for a life cycle of 1,000,000 cycles before refirbishment, and also for a shutter that will operate for up to 3 years before requiring maintenance. The present invention meets that need.
It has been found by the present inventors that the prior art shutters developed for reconnaissance cameras have inherent mechanical and electrical design limitations that prevent such a shutter design from meeting the life cycle, cycle rate and maintenance interval objectives required for high performance electro-optical reconnaissance cameras.
In particular, the present inventors have appreciated that the long-life, maintenance interval and cycle rates can be achieved by proving for digital signal processor (DSP) control of the motors moving the curtains, and by replacing the prior art DC motors and potentiometer curtain position sensing devices with AC motors having integral resolvers that provide motor shaft position feedback information. Furthermore, the mechanical clutch coupling the curtains together has been eliminated, with the precise movement of the curtains governed by the motors under control by the DSP. The marriage of DSP control with the AC motors and resolvers, providing direct coupling to the curtains and with built in resolver motor shaft position feedback, allows not only precise movement of the curtains and control of the exposure slit, but also provides durability, reliability, and low failure rates such that the shutter-meets the long-life, maintenance and frame rate requirements of current and next generation reconnaissance cameras.
Thus, in a first aspect we have invented an improvement to an electronic focal plane shutter having first and second curtains each having an edge, said edges defining an exposure slit for an image recording medium. The improvement comprises providing first and second motors having resolvers measuring the position of the shaft of the motors. The motors provide for movement of the first and second curtains. Additionally, the improvement includes a digital signal processor generating movement commands for the motors to effectuate movement of the curtains. The combination of the digital signal processor, motors and resolvers allows for precise movement of said first and second curtains and movement of the exposure slit over the image recording medium without requiring the use of a mechanical clutch to couple the first and second curtains, or analog potentiometers to sense the position of the curtains.